Polymer fibers such as hydrogel fibers are useful as carriers or scaffolds in various applications, for example, for cell transplantation or tissue engineering. It is desirable to seed cells in the fibers in some applications, such as in the fields of vascular and kidney tissue engineering. However, it remains a challenge to efficiently form polymer fibers with variable cross-sectional shapes and sizes, or to control the cell seeding in the fibers. Polymer fibers with seeded cells may be formed using an extrusion process but it is inconvenient to vary the fiber cross-sectional shape or size in a conventional extrusion process, or to control how cells are dispersed in the fiber.